Method of imaging coding pattern and identifying cell translations from different orientations

ABSTRACT

A method of imaging a coding pattern disposed on a surface of a substrate. The method comprises the steps of: (a) capturing an image of a portion of the coding pattern; (b) sampling a registration symbol contained in the imaged portion; (c) mapping a set of data elements to a first translation code symbol value when read in a first orientation or a second translation symbol code value when read in a second orientation; (d) determining a translation from the mapped set of data elements; and (e) using the translation to sample and decode tag data.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application claims the right of priority under 35 U.S.C.§119(e) based on U.S. Provisional Patent Application No. 60/974,077,filed Sep. 21, 2007 which is incorporated by reference herein in itsentirety as if fully set forth herein.

FIELD OF INVENTION

The present invention relates to a position-coding pattern on a surface.

CO-PENDING APPLICATIONS

The following applications have been filed by the Applicantsimultaneously with the present application:

NPT087US NPT088US NPT089US NPT090US NPT092US NPT093US NPT094US NPT095USNPT096US NPS151US NPP090US NPP091US NPP092US NPP093US NPP094US NPP095USNPP096US NPP097US NPS149US NPS150US NPZ032US NPZ033US NPP099US NPP100USNPP101US NPP102US NPP103US NPP104US NPP105US NPP106US NPP107US NPP108USThe disclosures of these co-pending applications are incorporated hereinby reference. The above applications have been identified by theirfiling docket number, which will be substituted with the correspondingapplication number, once assigned.

CROSS REFERENCES

The following patents or patent applications filed by the applicant orassignee of the present invention are hereby incorporated bycross-reference.

10/815621 10/815635 10/815647 11/488162 10/815636 11/041652 11/04160911/041556 10/815609 7204941 7278727 10/913380 7122076 7156289 09/5751976720985 7295839 09/722174 7068382 7094910 7062651 6644642 65499356987573 6727996 6760119 7064851 6290349 6428155 6785016 6831682 67418716965439 10/932044 6870966 6474888 6724374 6788982 7263270 67882936737591 09/693514 10/778056 10/778061 11/193482 7055739 6830196 71822477082562 10/409864 7108192 10/492169 10/492152 10/492168 10/4921617308148 6957768 7170499 11856061 11/672522 11/672950 11754310 120155077148345 12025746 12025762 12025765 10/407212 6902255 6755509

BACKGROUND

The Applicant has previously described a method of enabling users toaccess information from a computer system via a printed substrate e.g.paper. The substrate has a coding pattern printed thereon, which is readby an optical sensing device when the user interacts with the substrateusing the sensing device. A computer receives interaction data from thesensing device and uses this data to determine what action is beingrequested by the user. For example, a user may make handwritten inputonto a form or make a selection gesture around a printed item. Thisinput is interpreted by the computer system with reference to a pagedescription corresponding to the printed substrate.

It would desirable to improve the coding pattern on the substrate so asto maximize usage of images captured by the sensing device.

SUMMARY OF INVENTION

In a first aspect the present invention provides a substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by at least one        cell, each tag comprising respective tag data encoded by data        elements,

wherein each cell comprises a plurality of registration symbols encodedby a respective set of said data elements, each registration symbolidentifying a respective direction component of an orientation such thatsaid plurality of registration symbols in said cell together identifysaid orientation, wherein said orientation is an orientation of a layoutof said tag data with respect to said target grid.

Optionally, each cell comprises first and second orthogonal registrationsymbols, said first registration symbol identifying a first directioncomponent of said orientation, and said second registration symbolidentifying a second direction component of said orientation, such thatsaid first and second orthogonal registration symbols together identifysaid orientation.

Optionally, said set of data elements is represented by multi-pulseposition modulation.

Optionally, said set of data elements consists of m macrodots, each ofsaid macrodots occupying a respective position from a plurality ofpredetermined possible positions p within said cell, the respectivepositions of said macrodots representing one of a plurality of possibleregistration symbol values for said registration symbol.

Optionally, m is 2 or more and p>m.

Optionally, m is 2 and p is 6 so as to provide 15 possible registrationsymbol values.

Optionally, a plurality of said registration symbol values are mapped toa direction code symbol value, said direction code symbol valuerepresenting a direction component of said orientation.

Optionally, said orientation is one of four possible orientations, saidorientation being identifiable via a pair of 1-bit direction code symbolvalues.

Optionally, said each registration symbol value read in a firstorientation is reversed when read in an opposite second orientation.

Optionally, each registration symbol value maps to a “0” direction codesymbol value when read in said first orientation, and maps to a “1”direction code symbol value when read in said second orientation, suchthat determination of the orientation of said tag data is independent ofan orientation in which said registration symbol is read.

Optionally, each registration symbol further identifies at least one of:

-   -   a translation of said cell relative to a tag containing said        cell; and    -   a flag.

Optionally, each registration symbol value maps to an identical flagcode symbol value irrespective of an orientation of reading saidregistration symbol.

Optionally, each tag is square and comprises M² contiguous square cells,wherein M is an integer having a value of at least 2.

Optionally, M registration symbols in a row of M cells define a cyclicposition code having minimum distance M, said code being defined by afirst translation codeword.

Optionally, M registration symbols in a column of M cells define acyclic position code having minimum distance M, said code being definedby a second translation codeword.

Optionally, each tag comprises N cells, and at least N registrationsymbols form a third translation codeword with minimum distance N,wherein N is an integer having a value of at least 4.

Optionally, any tag-sized portion of said coding pattern is guaranteedto contain at least N registration symbols, thereby capturing said thirdtranslation codeword.

Optionally, each tag comprises N cells, and at least N firstregistration symbols form a first direction code with minimum distanceN, wherein N is an integer having a value of at least 4.

Optionally, at least N second registration symbols form a seconddirection code with minimum distance N, wherein N is an integer having avalue of at least 4.

Optionally, said cells are arranged such that any tag-sized portion ofsaid coding pattern guaranteed to contain said first and seconddirection codes.

In a second aspect the present invention provides a substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by a plurality of        contiguous cells, each tag comprising respective tag data        encoded by a respective set of said data elements,

wherein each cell comprises one or more registration symbols encoded bya respective set of said data elements, said one or more registrationsymbols identifying:

-   -   a translation of said cell relative to a tag containing said        cell; and    -   an orientation of a layout of said tag data with respect to said        target grid.

Optionally, each cell comprises a first and second registration symbols,said first registration symbol identifying a first orthogonaltranslation of said cell, said second registration symbol identifying asecond orthogonal translation of said cell.

Optionally, said first registration symbol identifies a first directioncomponent of said orientation, and said second registration symbolidentifies a second direction component of said orientation, such thatsaid first and second orthogonal registration symbols together identifysaid orientation via said first and second direction components.

Optionally, each tag is square and comprises M² contiguous square cells,wherein M is an integer having a value of at least 2.

Optionally, M registration symbols in a row of M cells define a cyclicposition code having minimum distance M, said code being defined by afirst translation codeword.

Optionally, M registration symbols in a column of M cells define acyclic position code having minimum distance M, said code being definedby a second translation codeword.

Optionally, each tag comprises N cells, and at least N registrationsymbols form a third translation codeword with minimum distance N,wherein N is an integer having a value of at least 4.

Optionally, any tag-sized portion of said coding pattern is guaranteedto contain at least N registration symbols, thereby capturing said thirdtranslation codeword.

Optionally, said orientation is one of four possible orientationsidentifiable via a pair of 1-bit direction code symbol values.

Optionally, each tag comprises N cells, and at least N firstregistration symbols form a first direction code with minimum distanceN, wherein N is an integer having a value of at least 4.

Optionally, at least N second registration symbols form a seconddirection code with minimum distance N, wherein N is an integer having avalue of at least 4.

Optionally, said orientation is identifiable from said first and seconddirection codes. Optionally, said cells are arranged such that anytag-sized portion of said coding pattern is guaranteed to contain saidfirst and second direction codes.

Optionally, said registration symbol further identifies a flag for saidtag.

Optionally, said data elements are macrodots.

Optionally, a portion of data is represented by m macrodots, each ofsaid macrodots occupying a respective position from a plurality ofpredetermined possible positions p within said cell, the respectivepositions of said macrodots representing one of a plurality of possibledata values.

Optionally, said portion of data is a Reed-Solomon symbol.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

Optionally, m is an integer of 2 or more, and p≧2m.

Optionally, said tag data is encoded as a local codeword comprised of aset of said Reed-Solomon symbols.

In a third aspect the present invention provides a method of imaging acoding pattern disposed on a surface of a substrate, said methodcomprising the steps of:

-   -   (a) capturing an image of a portion of said coding pattern, said        coding pattern comprising:        -   a plurality of target elements defining a target grid, said            target grid comprising a plurality of cells, wherein            neighboring cells share target elements;        -   a plurality of data elements contained in each cell; and        -   a plurality of tags, each tag being defined by at least one            cell, each tag comprising respective tag data encoded by a            respective set of said data elements,

wherein each cell comprises a plurality of registration symbols encodedby a respective set of said data elements, each registration symbolidentifying a respective direction component of an orientation of alayout of said tag data with respect to said target grid; and

-   -   (b) sampling and decoding a plurality of said registration        symbols contained in said imaged portion;    -   (c) determining, from the decoded registration symbols, the        orientation of the layout of the tag data relative to the target        grid; and    -   (d) using said determined orientation to sample and decode said        tag data.

Optionally, each cell comprises first and second orthogonal registrationsymbols, said first registration symbol identifying a first directioncomponent of said orientation, and said second registration symbolidentifying a second direction component of said orientation, such thatsaid first and second orthogonal registration symbols together identifysaid orientation.

Optionally, said orientation is one of four possible orientations, saidorientation being identifiable via a pair of 1-bit orthogonal directioncomponents.

Optionally, each tag is defined by a plurality of contiguous cellsOptionally, each registration symbol further identifies:

-   -   a translation of said cell relative to a tag containing said        cell.

Optionally, each tag comprises N cells, and at least N firstregistration symbols form a first direction code with minimum distanceN, wherein N is an integer having a value of at least 4.

Optionally, at least N second registration symbols form a seconddirection code with minimum distance N, wherein N is an integer having avalue of at least 4.

Optionally, said cells are arranged such that any tag-sized portion ofsaid coding pattern is guaranteed to contain said first and seconddirection codes.

Optionally, each registration symbol further identifies a flag for saidtag.

Optionally, the method further comprising the step of identifying aposition using the decoded tag data.

In a further aspect the present invention provides a system for imaginga coding pattern disposed on a surface of a substrate, said systemcomprising:

(A) said substrate, wherein said coding pattern comprises:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by at least one        cell, each tag comprising respective tag data encoded by a        respective set of said data elements,

wherein each cell comprises a plurality of registration symbols encodedby a respective set of said data elements, each registration symbolidentifying a respective direction component of an orientation of alayout of said tag data with respect to said target grid; and

(B) an optical reader comprising:

-   -   an image sensor for capturing an image of a portion of said        coding pattern; and    -   a processor configured for:        -   (i) sampling and decoding a plurality of said registration            symbols contained in said imaged portion;        -   (ii) determining, from the decoded registration symbols, the            orientation of the layout of the tag data relative to the            target grid; and        -   (iii) using said determined orientation to sample and decode            said tag data.

Optionally, each cell comprises first and second orthogonal registrationsymbols, said first registration symbol identifying a first directioncomponent of said orientation, and said second registration symbolidentifying a second direction component of said orientation, such thatsaid first and second orthogonal registration symbols together identifysaid orientation.

Optionally, said orientation is one of four possible orientations, saidorientation being identifiable via a pair of 1-bit orthogonal directioncomponents.

Optionally, each tag is defined by a plurality of contiguous cells

Optionally, each registration symbol further identifies:

-   -   a translation of said cell relative to a tag containing said        cell.

Optionally, each tag comprises N cells, and at least N firstregistration symbols form a first direction code with minimum distanceN, wherein N is an integer having a value of at least 4.

Optionally, at least N second registration symbols form a seconddirection code with minimum distance N, wherein N is an integer having avalue of at least 4.

Optionally, said cells are arranged such that any tag-sized portion ofsaid coding pattern is guaranteed to contain said first and seconddirection codes.

Optionally, each registration symbol further identifies a flag for saidtag.

Optionally, said processor is further configured for:

-   -   identifying a position using the decoded tag data.

In a fourth aspect the present invention provides a method of imaging acoding pattern disposed on a surface of a substrate, said methodcomprising the steps of:

-   -   (a) capturing an image of a portion of said coding pattern, said        coding pattern comprising:        -   a plurality of target elements defining a target grid, said            target grid comprising a plurality of cells, wherein            neighboring cells share target elements;        -   a plurality of data elements contained in each cell; and        -   a plurality of tags, each tag being defined by a plurality            of contiguous cells, each tag comprising respective tag data            encoded by a respective set of said data elements,

wherein each cell comprises one or more registration symbols encoded bya respective set of said data elements, said one or more registrationsymbols identifying:

-   -   a translation of said cell relative to a tag containing said        cell; and    -   an orientation of a layout of said tag data with respect to said        target grid;    -   (b) sampling and decoding a plurality of said registration        symbols contained in said imaged portion;    -   (c) determining, from the decoded registration symbols, said        orientation and said translation; and    -   (d) using said orientation and said translation to sample and        decode said tag data.

Optionally, each cell comprises a first and second registration symbols,said first registration symbol identifying a first orthogonaltranslation of said cell, said second registration symbol identifying asecond orthogonal translation of said cell, and wherein said imagedportion contains both first and second registration symbols.

Optionally, said first registration symbol identifies a first directioncomponent of said orientation, and said second registration symbolidentifies a second direction component of said orientation, such thatsaid first and second orthogonal registration symbols together identifysaid orientation via said first and second direction components.

Optionally, each tag is square and comprises M² contiguous square cells,wherein M is an integer having a value of at least 2.

Optionally, M registration symbols in a row of M cells define a cyclicposition code having minimum distance M, said code being defined by afirst translation codeword.

Optionally, M registration symbols in a column of M cells define acyclic position code having minimum distance M, said code being definedby a second translation codeword.

Optionally, each tag comprises N cells, and at least N registrationsymbols form a third translation codeword with minimum distance N,wherein N is an integer having a value of at least 4.

Optionally, said imaged portion of said coding pattern is guaranteed tocontain at least N registration symbols, thereby capturing said thirdtranslation codeword.

Optionally, said orientation is one of four possible orientationsidentifiable via a pair of 1-bit direction components.

Optionally, said registration symbol further identifies a flag for saidtag, and said flag is used to sample and decode said tag data.

In a further aspect the present invention provides a system for imaginga coding pattern disposed on a surface of a substrate, said systemcomprising:

(A) said substrate, wherein said coding pattern comprises:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by at least one        cell, each tag comprising respective tag data encoded by a        respective set of said data elements,

wherein each cell comprises a plurality of registration symbols encodedby a respective set of said data elements, each registration symbolidentifying a respective direction component of an orientation of alayout of said tag data with respect to said target grid; and

(B) an optical reader comprising:

-   -   an image sensor for capturing an image of a portion of said        coding pattern; and    -   a processor configured for:        -   (i) sampling and decoding a plurality of said registration            symbols contained in said imaged portion;        -   (ii) determining, from the decoded registration symbols, the            orientation of the layout of the tag data relative to the            target grid; and        -   (iii) using said determined orientation to sample and decode            said tag data.

Optionally, each cell comprises first and second orthogonal registrationsymbols, said first registration symbol identifying a first directioncomponent of said orientation, and said second registration symbolidentifying a second direction component of said orientation, such thatsaid first and second orthogonal registration symbols together identifysaid orientation.

Optionally, said orientation is one of four possible orientations, saidorientation being identifiable via a pair of 1-bit orthogonal directioncomponents.

Optionally, each tag is defined by a plurality of contiguous cells

Optionally, each registration symbol further identifies:

-   -   a translation of said cell relative to a tag containing said        cell.

Optionally, each tag comprises N cells, and at least N firstregistration symbols form a first direction code with minimum distanceN, wherein N is an integer having a value of at least 4.

Optionally, at least N second registration symbols form a seconddirection code with minimum distance N, wherein N is an integer having avalue of at least 4.

Optionally, said cells are arranged such that any tag-sized portion ofsaid coding pattern is guaranteed to contain said first and seconddirection codes.

Optionally, each registration symbol further identifies a flag for saidtag.

Optionally, said processor is further configured for:

-   -   identifying a position using the decoded tag data.

In a fifth aspect the present invention provides a substrate having acoding pattern disposed on a surface thereof, said coding patterncomprising:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;

a plurality of data elements contained in each cell; and

a plurality of tags, each tag being defined by a plurality of contiguouscells, each tag comprising respective tag data encoded by data elements,

wherein each cell comprises at least one registration symbol encoded bya respective set of said data elements, said set of data elementsidentifying a translation of said cell relative to a tag containing saidcell,

wherein each set of data elements maps to a first translation codesymbol value when read in a first orientation, and maps to a secondtranslation code symbol value when read in a second orientation which isdifferent from said first orientation, such that said translation ofsaid cell relative to said tag is identifiable in either of said firstor second orientations.

Optionally, said second orientation is rotated 180 degrees from saidfirst orientation.

Optionally, said set of data elements is represented by multi-pulseposition modulation.

Optionally, said set of data elements consists of m macrodots, each ofsaid macrodots occupying a respective position from a plurality ofpredetermined possible positions p within said cell, the respectivepositions of said macrodots representing one of a plurality of possibleregistration symbol values for said registration symbol.

Optionally, m is 2 or more and p>m.

Optionally, m is 2 and p is 6 so as to provide 15 possible registrationsymbol values.

Optionally, a plurality of said registration symbol values are mapped toeach translation code symbol value.

Optionally, said second orientation reverses each registration symbolvalue with respect to said first orientation.

Optionally, each tag contains an odd number of cells aligned in a row orcolumn, and wherein a registration symbol contained in a central cell ofsaid row or column is represented by a registration symbol value whichmaps to identical first and second translation code symbol values.

Optionally, each tag contains a plurality of cells aligned in a row orcolumn, each row or column having a first cell at one end and a secondcell at an opposite end, said first cell containing a first registrationsymbol represented by a first registration symbol value which maps to afirst translation symbol code value in said first orientation, saidsecond cell containing a second registration symbol represented by asecond registration symbol value which maps to a second translation codesymbol value in said first orientation, wherein said first translationcode symbol value and said second translation code symbol value areinterchanged in said second orientation.

Optionally, each registration symbol further identifies at least one of:

-   -   an orientation of a layout of said tag data with respect to said        target grid;    -   a direction component of said orientation; and    -   a flag code.

Optionally, each cell comprises first and second registration symbols,said first registration symbol identifying a first orthogonaltranslation of said cell, said second registration symbol identifying asecond orthogonal translation of said cell.

Optionally, each tag is square and comprises M² contiguous square cells,wherein M is an integer having a value of at least 2.

Optionally, M registration symbols in a row of M cells define a cyclicposition code having minimum distance M, said code being defined by afirst translation codeword.

Optionally, M registration symbols in a column of M cells define acyclic position code having minimum distance M, said code being definedby a second translation codeword.

Optionally, each tag comprises N cells, and at least N registrationsymbols form a third translation codeword with minimum distance N,wherein N is an integer having a value of at least 4.

Optionally, any tag-sized portion of said coding pattern is guaranteedto contain at least N registration symbols, thereby capturing said thirdtranslation codeword.

Optionally, said tag data is encoded by one or more Reed-Solomonsymbols.

Optionally, each cell defines a symbol group, each symbol groupcomprising a plurality of Reed-Solomon symbols encoded by a plurality ofsaid data elements.

Optionally, said tag data is encoded as a local codeword comprised of aset of said Reed-Solomon symbols.

In a sixth aspect the present invention provides a method of imaging acoding pattern disposed on a surface of a substrate, said methodcomprising the steps of:

-   -   (a) capturing an image of a portion of said coding pattern, said        coding pattern comprising:        -   a plurality of target elements defining a target grid, said            target grid comprising a plurality of cells, wherein            neighboring cells share target elements;        -   a plurality of data elements contained in each cell; and        -   a plurality of tags, each tag being defined by a plurality            of contiguous cells, each tag comprising respective tag data            encoded by data elements,

wherein each cell comprises at least one registration symbol encoded bya respective set of said data elements, said set of data elementsidentifying a translation of said cell relative to a tag containing saidcell;

-   -   (b) sampling one or more registration symbols contained in said        imaged portion;    -   (c) mapping each set of data elements to a first translation        code symbol value when read in a first orientation or a second        translation symbol code value when read in a second orientation;    -   (d) determining the translation from the mapped set of data        elements; and    -   (e) using the translation to sample and decode said tag data.

Optionally, second orientation is rotated 180 degrees from said firstorientation.

Optionally, said set of data elements is represented by multi-pulseposition modulation.

Optionally, said set of data elements consists of m macrodots, each ofsaid macrodots occupying a respective position from a plurality ofpredetermined possible positions p within said cell, the respectivepositions of said macrodots representing one of a plurality of possibleregistration symbol values for said registration symbol.

Optionally, m is 2 and p is 6 so as to provide 15 possible registrationsymbol values.

Optionally, a plurality of said registration symbol values are mapped toeach translation code value.

Optionally, said second orientation reverses each registration symbolvalue with respect to said first orientation.

Optionally, each tag contains an odd number of cells aligned in a row orcolumn, and wherein a registration symbol contained in a central cell ofsaid row or column is represented by a registration symbol value whichmaps to identical first and second translation code symbol values.

Optionally, each tag contains a plurality of cells aligned in a row orcolumn, each row or column having a first cell at one end and a secondcell at an opposite end, said first cell containing a first registrationsymbol represented by a first registration symbol value which maps to afirst translation symbol code value in said first orientation, saidsecond cell containing a second registration symbol represented by asecond registration symbol value which maps to a second translation codesymbol value in said first orientation, wherein said first translationcode symbol value and said second translation code symbol value areinterchanged in said second orientation.

Optionally, the method further comprising the step of decoding, fromeach registration symbol, at least one of:

-   -   an orientation of a layout of said tag data with respect to said        target grid;    -   a direction component of said orientation; and    -   a flag code.

In another aspect the present invention provides a system for imaging acoding pattern disposed on a surface of a substrate, said systemcomprising:

(A) said substrate, wherein said coding pattern comprises:

-   -   a plurality of target elements defining a target grid, said        target grid comprising a plurality of cells, wherein neighboring        cells share target elements;    -   a plurality of data elements contained in each cell; and    -   a plurality of tags, each tag being defined by a plurality of        contiguous cells, each tag comprising respective tag data        encoded by data elements,

wherein each cell comprises at least one registration symbol encoded bya respective set of said data elements, said set of data elementsidentifying a translation of said cell relative to a tag containing saidcell; and

(B) an optical reader comprising:

-   -   an image sensor for capturing an image of a portion of said        coding pattern; and    -   a processor configured for:        -   (i) sampling one or more registration symbols contained in            said imaged portion;        -   (ii) mapping each set of data elements to a first            translation code symbol value when read in a first            orientation or a second translation code symbol value when            read in a second orientation;        -   (iii) determining the translation from the mapped set of            data elements; and        -   (iv) using the translation to sample and decode said tag            data.

Optionally, second orientation is rotated 180 degrees from said firstorientation.

Optionally, said set of data elements is represented by multi-pulseposition modulation.

Optionally, said set of data elements consists of m macrodots, each ofsaid macrodots occupying a respective position from a plurality ofpredetermined possible positions p within said cell, the respectivepositions of said macrodots representing one of a plurality of possibleregistration symbol values for said registration symbol.

Optionally, m is 2 or more and p>m.

Optionally, a plurality of said registration symbol values are mapped toeach translation code value.

Optionally, said second orientation reverses each registration symbolvalue with respect to said first orientation.

Optionally, each tag contains an odd number of cells aligned in a row orcolumn, and wherein a registration symbol contained in a central cell ofsaid row or column is represented by a registration symbol value whichmaps to identical first and second translation code symbol values.

Optionally, each tag contains a plurality of cells aligned in a row orcolumn, each row or column having a first cell at one end and a secondcell at an opposite end, said first cell containing a first registrationsymbol represented by a first registration symbol value which maps to afirst translation symbol code value in said first orientation, saidsecond cell containing a second registration symbol represented by asecond registration symbol value which maps to a second translation codesymbol value in said first orientation, wherein said first translationcode symbol value and said second translation code symbol value areinterchanged in said second orientation.

Optionally, said processor is further configured for decoding, from eachregistration symbol, at least one of:

-   -   an orientation of a layout of said tag data with respect to said        target grid;    -   a direction component of said orientation; and    -   a flag code.

BRIEF DESCRIPTION OF DRAWINGS

Preferred and other embodiments of the invention will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic of a the relationship between a sample printednetpage and its online page description;

FIG. 2 shows an embodiment of basic netpage architecture with variousalternatives for the relay device;

FIG. 3 shows the structure of a tag;

FIG. 4 shows a group of twelve data symbols and four targets;

FIG. 5 shows the layout of a 2-6PPM and 3-6PPM data symbol;

FIG. 6 shows the spacing of macrodot positions;

FIG. 7 shows the layout of a 2-6PPM registration symbol;

FIG. 8 shows a semi-replicated x-coordinate codeword X;

FIG. 9 shows a semi-replicated y-coordinate codeword Y;

FIG. 10 shows common codewords A, B, C and D, with codeword A shown inbold outline;

FIG. 11 shows an optional codeword E;

FIG. 12 shows the layout of a complete tag;

FIG. 13 shows the layout of a Reed-Solomon codeword;

FIG. 14 is a flowchart of image processing;

FIG. 15 shows a nib and elevation of the pen held by a user;

FIG. 16 shows the pen held by a user at a typical incline to a writingsurface;

FIG. 17 is a lateral cross section through the pen;

FIG. 18A is a bottom and nib end partial perspective of the pen;

FIG. 18B is a bottom and nib end partial perspective with the fields ofillumination and field of view of the sensor window shown in dottedoutline;

FIG. 19 is a longitudinal cross section of the pen;

FIG. 20A is a partial longitudinal cross section of the nib and barrelmolding;

FIG. 20B is a partial longitudinal cross section of the IR LED's and thebarrel molding;

FIG. 21 is a ray trace of the pen optics adjacent a sketch of the inkcartridge;

FIG. 22 is a side elevation of the lens;

FIG. 23 is a side elevation of the nib and the field of view of theoptical sensor; and

FIG. 24 is a block diagram of the pen electronics.

DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS 1.1 NetpageSystem Architecture

In a preferred embodiment, the invention is configured to work with thenetpage networked computer system, a detailed overview of which follows.It will be appreciated that not every implementation will necessarilyembody all or even most of the specific details and extensions discussedbelow in relation to the basic system. However, the system is describedin its most complete form to reduce the need for external reference whenattempting to understand the context in which the preferred embodimentsand aspects of the present invention operate.

In brief summary, the preferred form of the netpage system employs acomputer interface in the form of a mapped surface, that is, a physicalsurface which contains references to a map of the surface maintained ina computer system. The map references can be queried by an appropriatesensing device. Depending upon the specific implementation, the mapreferences may be encoded visibly or invisibly, and defined in such away that a local query on the mapped surface yields an unambiguous mapreference both within the map and among different maps. The computersystem can contain information about features on the mapped surface, andsuch information can be retrieved based on map references supplied by asensing device used with the mapped surface. The information thusretrieved can take the form of actions which are initiated by thecomputer system on behalf of the operator in response to the operator'sinteraction with the surface features.

In its preferred form, the netpage system relies on the production of,and human interaction with, netpages. These are pages of text, graphicsand images printed on ordinary paper, but which work like interactivewebpages. Information is encoded on each page using ink which issubstantially invisible to the unaided human eye. The ink, however, andthereby the coded data, can be sensed by an optically imaging sensingdevice and transmitted to the netpage system. The sensing device maytake the form of a clicker (for clicking on a specific position on asurface), a pointer having a stylus (for pointing or gesturing on asurface using pointer strokes), or a pen having a marking nib (formarking a surface with ink when pointing, gesturing or writing on thesurface). References herein to “pen” or “netpage pen” are provided byway of example only. It will, of course, be appreciated that the pen maytake the form of any of the sensing devices described above.

In one embodiment, active buttons and hyperlinks on each page can beclicked with the sensing device to request information from the networkor to signal preferences to a network server. In one embodiment, textwritten by hand on a netpage is automatically recognized and convertedto computer text in the netpage system, allowing forms to be filled in.In other embodiments, signatures recorded on a netpage are automaticallyverified, allowing e-commerce transactions to be securely authorized. Inother embodiments, text on a netpage may be clicked or gestured toinitiate a search based on keywords indicated by the user.

As illustrated in FIG. 1, a printed netpage 1 can represent ainteractive form which can be filled in by the user both physically, onthe printed page, and “electronically”, via communication between thepen and the netpage system. The example shows a “Request” formcontaining name and address fields and a submit button. The netpage 1consists of graphic data 2, printed using visible ink, and a surfacecoding pattern 3 superimposed with the graphic data. The surface codingpattern 3 comprises a collection of tags 4. One such tag 4 is shown inthe shaded region of FIG. 1, although it will be appreciated thatcontiguous tags 4, defined by the coding pattern 3, are densely tiledover the whole netpage 1.

The corresponding page description 5, stored on the netpage network,describes the individual elements of the netpage. In particular itdescribes the type and spatial extent (zone) of each interactive element(i.e. text field or button in the example), to allow the netpage systemto correctly interpret input via the netpage. The submit button 6, forexample, has a zone 7 which corresponds to the spatial extent of thecorresponding graphic 8.

As illustrated in FIG. 2, a netpage sensing device 400, such as the pendescribed in Section 3, works in conjunction with a netpage relay device601, which is an Internet-connected device for home, office or mobileuse. The pen 400 is wireless and communicates securely with the netpagerelay device 601 via a short-range radio link 9. In an alternativeembodiment, the netpage pen 400 utilises a wired connection, such as aUSB or other serial connection, to the relay device 601.

The relay device 601 performs the basic function of relaying interactiondata to a page server 10, which interprets the interaction data. Asshown in FIG. 2, the relay device 601 may, for example, take the form ofa personal computer 601 a, a netpage printer 601 b or some other relay601 c (e.g. personal computer or mobile phone incorporating a webbrowser).

The netpage printer 601 b is able to deliver, periodically or on demand,personalized newspapers, magazines, catalogs, brochures and otherpublications, all printed at high quality as interactive netpages.Unlike a personal computer, the netpage printer is an appliance whichcan be, for example, wall-mounted adjacent to an area where the morningnews is first consumed, such as in a user's kitchen, near a breakfasttable, or near the household's point of departure for the day. It alsocomes in tabletop, desktop, portable and miniature versions. Netpagesprinted on-demand at their point of consumption combine the ease-of-useof paper with the timeliness and interactivity of an interactive medium.

Alternatively, the netpage relay device 601 may be a portable device,such as a mobile phone or PDA, a laptop or desktop computer, or aninformation appliance connected to a shared display, such as a TV. Ifthe relay device 601 is not a netpage printer 601 b which printsnetpages digitally and on demand, the netpages may be printed bytraditional analog printing presses, using such techniques as offsetlithography, flexography, screen printing, relief printing androtogravure, as well as by digital printing presses, using techniquessuch as drop-on-demand inkjet, continuous inkjet, dye transfer, andlaser printing.

As shown in FIG. 2, the netpage sensing device 400 interacts with aportion of the tag pattern on a printed netpage 1, or other printedsubstrate such as a label of a product item 251, and communicates, via ashort-range radio link 9, the interaction to the relay device 601. Therelay 601 sends corresponding interaction data to the relevant netpagepage server 10 for interpretation. Raw data received from the sensingdevice 400 may be relayed directly to the page server 10 as interactiondata. Alternatively, the interaction data may be encoded in the form ofan interaction URI and transmitted to the page server 10 via a user'sweb browser 601 c. The web browser 601 c may then receive a URI from thepage server 10 and access a webpage via a webserver 201. In somecircumstances, the page server 10 may access application computersoftware running on a netpage application server 13.

The netpage relay device 601 can be configured to support any number ofsensing devices, and a sensing device can work with any number ofnetpage relays. In the preferred implementation, each netpage sensingdevice 400 has a unique identifier. This allows each user to maintain adistinct profile with respect to a netpage page server 10 or applicationserver 13.

Digital, on-demand delivery of netpages 1 may be performed by thenetpage printer 601 b, which exploits the growing availability ofbroadband Internet access. Netpage publication servers 14 on the netpagenetwork are configured to deliver print-quality publications to netpageprinters. Periodical publications are delivered automatically tosubscribing netpage printers via pointcasting and multicasting Internetprotocols. Personalized publications are filtered and formattedaccording to individual user profiles.

A netpage pen may be registered with a netpage registration server 11and linked to one or more payment card accounts. This allows e-commercepayments to be securely authorized using the netpage pen. The netpageregistration server compares the signature captured by the netpage penwith a previously registered signature, allowing it to authenticate theuser's identity to an e-commerce server. Other biometrics can also beused to verify identity. One version of the netpage pen includesfingerprint scanning, verified in a similar way by the netpageregistration server.

1.2 Netpages

Netpages are the foundation on which a netpage network is built. Theyprovide a paper-based user interface to published information andinteractive services.

As shown in FIG. 1, a netpage consists of a printed page (or othersurface region) invisibly tagged with references to an onlinedescription 5 of the page. The online page description 5 is maintainedpersistently by the netpage page server 10. The page descriptiondescribes the visible layout and content of the page, including text,graphics and images. It also describes the input elements on the page,including buttons, hyperlinks, and input fields. A netpage allowsmarkings made with a netpage pen on its surface to be simultaneouslycaptured and processed by the netpage system.

Multiple netpages (for example, those printed by analog printingpresses) can share the same page description. However, to allow inputthrough otherwise identical pages to be distinguished, each netpage maybe assigned a unique page identifier. This page ID has sufficientprecision to distinguish between a very large number of netpages.

Each reference to the page description 5 is repeatedly encoded in thenetpage pattern. Each tag (and/or a collection of contiguous tags)identifies the unique page on which it appears, and thereby indirectlyidentifies the page description 5. Each tag also identifies its ownposition on the page. Characteristics of the tags are described in moredetail below.

Tags are typically printed in infrared-absorptive ink on any substratewhich is infrared-reflective, such as ordinary paper, or in infraredfluorescing ink. Near-infrared wavelengths are invisible to the humaneye but are easily sensed by a solid-state image sensor with anappropriate filter.

A tag is sensed by a 2D area image sensor in the netpage sensing device,and the tag data is transmitted to the netpage system via the nearestnetpage relay device 601. The pen 400 is wireless and communicates withthe netpage relay device 601 via a short-range radio link. It isimportant that the pen recognize the page ID and position on everyinteraction with the page, since the interaction is stateless. Tags areerror-correctably encoded to make them partially tolerant to surfacedamage.

The netpage page server 10 maintains a unique page instance for eachunique printed netpage, allowing it to maintain a distinct set ofuser-supplied values for input fields in the page description 5 for eachprinted netpage 1.

2 Netpage Tags 2.1 Tag Data Content

Each tag 4 identifies an absolute location of that tag within a regionof a substrate.

Each interaction with a netpage should also provide a region identitytogether with the tag location. In a preferred embodiment, the region towhich a tag refers coincides with an entire page, and the region ID istherefore synonymous with the page ID of the page on which the tagappears. In other embodiments, the region to which a tag refers can bean arbitrary subregion of a page or other surface. For example, it cancoincide with the zone of an interactive element, in which case theregion ID can directly identify the interactive element.

As described in the Applicant's previous applications (e.g. U.S. Pat.No. 6,832,717), the region identity may be encoded discretely in eachtag 4. As will be described in more detail below, the region identitymay be encoded by a plurality of contiguous tags in such a way thatevery interaction with the substrate still identifies the regionidentity, even if a whole tag is not in the field of view of the sensingdevice.

Each tag 4 should preferably identify an orientation of the tag relativeto the substrate on which the tag is printed. Orientation data read froma tag enables the rotation (yaw) of the pen 101 relative to thesubstrate to be determined

A tag 4 may also encode one or more flags which relate to the region asa whole or to an individual tag. One or more flag bits may, for example,signal a sensing device to provide feedback indicative of a functionassociated with the immediate area of the tag, without the sensingdevice having to refer to a description of the region. A netpage penmay, for example, illuminate an “active area” LED when in the zone of ahyperlink.

A tag 4 may also encode a digital signature or a fragment thereof. Tagsencoding (partial) digital signatures are useful in applications whereit is required to verify a product's authenticity. Such applications aredescribed in, for example, US Publication No. 2007/0108285, the contentsof which is herein incorporated by reference. The digital signature maybe encoded in such a way that it can be retrieved from every interactionwith the substrate. Alternatively, the digital signature may be encodedin such a way that it can be assembled from a random or partial scan ofthe substrate.

It will, of course, be appreciated that other types of information (e.g.tag size etc) may also be encoded into each tag or a plurality of tags,as will be explained in more detail below.

2.2 General Tag Structure

As described above in connection with FIG. 1, the netpage surface codinggenerally consists of a dense planar tiling of tags. In the presentinvention, each tag 4 is represented by a coding pattern which containstwo kinds of elements. Referring to FIGS. 3 and 4, the first kind ofelement is a target element. Target elements in the form of target dots301 allow a tag 4 to be located in an image of a coded surface, andallow the perspective distortion of the tag to be inferred. The secondkind of element is a data element in the form of a macrodot 302 (seeFIG. 6). The macrodots 302 encode data values. As described in theApplicant's earlier disclosures (e.g. U.S. Pat. No. 6,832,717), thepresence or absence of a macrodot was be used to represent a binary bit.However, the tag structure of the present invention encodes a data valueusing multi-pulse position modulation, which is described in more detailin Section 2.3.

The coding pattern 3 is represented on the surface in such a way as toallow it to be acquired by an optical imaging system, and in particularby an optical system with a narrowband response in the near-infrared.The pattern 3 is typically printed onto the surface using a narrowbandnear-infrared ink.

FIG. 3 shows the structure of a complete tag 4 with target elements 301shown. The tag 4 is square and contains sixteen target elements. Thosetarget elements 301 located at the edges and corners of the tag (twelvein total) are shared by adjacent tags and define the perimeter of thetag. In contrast with the Applicant's previous tag designs, the highnumber of target elements 301 advantageously facilitates accuratedetermination of a perspective distortion of the tag 4 when it is imagedby the sensing device 101. This improves the accuracy of tag sensingand, ultimately, position determination.

The tag 4 consists of a square array of nine symbol groups 303. Symbolgroups 303 are demarcated by the target elements 301 so that each symbolgroup is contained within a square defined by four target elements.Adjacent symbol groups 303 are contiguous and share targets.

Since the target elements 301 are all identical, they do not demarcateone tag from its adjacent tags. Viewed purely at the level of targetelements, only symbol groups 303, which define cells of a target grid,can be distinguished—the tags 4 themselves are indistinguishable byviewing only the target elements. Hence, tags 4 must be aligned with thetarget grid as part of tag decoding.

The tag 4 is designed to allow all tag data, with the exception of anembedded data object (see Section 2.9.3), to be recovered from animaging field of view substantially the size of the tag. This impliesthat any data unique to the tag 4 must appear four times within thetag—i.e. once in each quadrant or quarter; any data unique to a columnor row of tags must appear twice within the tag—i.e. once in eachhorizontal half or vertical half of the tag respectively; and any datacommon to a set of tags needs to appear once within the tag.

2.3 Symbol Groups

As shown in FIG. 4, each of the nine symbol groups 303 comprises twelvedata symbols 304, each data symbol being part of a codeword. Inaddition, each symbol group 303 comprises a pair of registrationsymbols—a vertical registration symbol (‘VRS’) and a horizontalregistration symbol (‘HRS’). These allow the orientation and/ortranslation of the tag in the field of view to be determined.Translation refers to the translation of tag(s) relative to the symbolgroups 303 in the field of view. In other words, the registrationsymbols enable alignment of the ‘invisible’ tags with the target grid.

Each data symbol 304 is a multi-pulse position modulated (PPM) datasymbol. Typically, each PPM data symbol 304 encodes a single 4-bitReed-Solomon symbol using either 2 or 3 macrodots in any of 6 positions{p₀, p₁, p₂, p₃, p₄, p₅}, i.e. using 2-6 or 3-6 pulse-positionmodulation (PPM). However, it will be appreciated that other forms ofmulti-PPM encoding are equally possible.

3-6PPM has a range of 20 codes, or 4.3 bits, and is used forReed-Solomon redundancy symbols. 2-6PPM has a range of 15 codes, or 3.9bits, and is used for Reed-Solomon data symbols. 4-bit Reed-Solomon datasymbols are converted to base 15 prior to encoding to reduce the numberof required codes per symbol to 15.

FIG. 5 shows the layout for a 2-6PPM or 3-6PPM data symbol 304.

Table 1 defines the mapping from 2-6PPM symbol values to Reed-Solomonsymbol values.

TABLE 1 2-6 PPM to Reed-Solomon symbol mapping CorrespondingReed-Solomon 2-6 PPM symbol symbol value value (p₅-p₀) (base 15) 0000110 000101 1 000110 2 001001 3 001010 4 001100 5 010001 6 010010 7 0101008 011000 9 100001 a 100010 b 100100 c 101000 d 110000 e

Table 2 defines the mapping from 3-6PPM symbol values to Reed-Solomonsymbol values. Unused symbol values can be treated as erasures.

TABLE 2 3-6 PPM to Reed-Solomon symbol mapping CorrespondingReed-Solomon 3-6 PPM symbol symbol value value (p₅-p₀) (base 16) 000111unused 001011 unused 001101 0 001110 1 010011 2 010101 3 010110 4 0110015 011010 6 011100 7 100011 8 100101 9 100110 a 101001 b 101010 c 101100d 110001 e 110010 f 110100 unused 111000 unused

2.4 Targets and Macrodots

The spacing of macrodots 302 in both dimensions, as shown in FIG. 6, isspecified by the parameter s. It has a nominal value of 127 μm, based on8 dots printed at a pitch of 1600 dots per inch.

Only macrodots 302 are part of the representation of a symbol 304 in thepattern. The outline of a symbol 304 is shown in, for example, FIGS. 3and 4 merely to elucidate more clearly the structure of a tag 4.

A macrodot 302 is nominally square with a nominal size of ( 4/8)s.However, it is allowed to vary in size by ±10% according to thecapabilities of the device used to produce the pattern.

A target 301 is nominally circular with a nominal diameter of ( 12/8)s.However, it is allowed to vary in size by ±10% according to thecapabilities of the device used to produce the pattern.

Each symbol group 303 has a width of 10s. Therefore, each tag 4 has awidth of 30s and a length of 30s. However, it should be noted from FIG.3 that the tag 4 is configured so that some data symbols 304A extendbeyond the perimeter edge of the tag 4 by one macrodot unit (1s), andinterlock with complementary symbol groups 304B from adjacent tags. Thisarrangement provides a tessellated pattern of data symbols 304 withinthe target grid. From a data acquisition standpoint, tessellation ofdata symbols in this way increases the effective length of each tag 4 byone macrodot unit.

The macrodot spacing, and therefore the overall scale of the tagpattern, is allowed to vary by 127 μm and 120 μm according to thecapabilities of the device used to produce the pattern. Any deviationfrom the nominal scale is recorded in each tag (in a tag size ID field)to allow accurate generation of position samples.

These tolerances are independent of one another. They may be refinedwith reference to particular printer characteristics.

2.5 Field of View

As mentioned above, the tag 4 is designed to allow all tag data to berecovered from an imaging field of view roughly the size of the tag. Anydata common to a set of contiguous tags only needs to appear once withineach tag, since fragments of the common data can be recovered fromadjacent tags. Any data common only to a column or row of tags mustappear twice within the tag—i.e. once in each horizontal half orvertical half of the tag respectively. And any data unique to the tagmust appear four times within the tag—i.e. once in each quadrant.

Although data which is common to a set of tags, in one or both spatialdimensions, may be decoded from fragments from adjacent tags,pulse-position modulated values are best decoded from spatially-coherentsamples, since this allows raw sample values to be compared withoutfirst being normalised. This implies that the field of view must belarge enough to contain two complete copies of each such pulse-positionmodulated value. The tag is designed so that the maximum extent of apulse-position modulated value is three macrodots. Making the field ofview at least as large as the tag plus three macrodot units guaranteesthat pulse-position modulated values can be coherently sampled.

The only exceptions are the translation codes described in the nextsection, which are four macrodot units long. However, these are highlyredundant and the loss of up to four symbols at the edge of the field ofview is not a problem.

2.6 Encoded Codes and Codewords

In this following section (Section 2.6), each symbol in FIGS. 8 to 12 isshown with a unique label. The label consists of an alphabetic prefixwhich identifies which codeword the symbol is part of, and a numericsuffix which indicates the index of the symbol within the codeword. Forsimplicity only data symbols 304 are shown, not registration symbols.

Although some symbol labels are shown rotated to indicate the symmetryof the layout of certain codewords, the layout of each symbol isdetermined by its position within a symbol group and not by the rotationof the symbol label (as described in, for example, the Applicant's USPublication No. 2006/146069).

2.6.1 Registration Symbols

Each registration symbol is encoded using 2-6PPM. FIG. 7 shows thelayout of the registration symbol.

As shown in FIG. 4, the horizontal and vertical registration symbolseach appear once within a symbol group. The registration symbols of anentire tag typically indicate the vertical and horizontal translation ofthe tag by coding two orthogonal translation codes, and the orientationof the tag by coding two orthogonal direction codes.

Each registration symbol also encodes a one-bit symbol of a flag code(see Section 2.6.2).

Table 3 defines the mapping from 2-6PPM registration symbol values toflag code, direction code and translation code symbol values.

TABLE 3 2-6 PPM registration symbol values to flag code, direction codeand translation code symbol mapping 2-6 PPM direction translation symbolvalue flag code code symbol code symbol {p₅-p₀} symbol value value value001, 001 0 0 0 000, 011 1 100, 010 0 1 011, 000 1 001, 010 0 0 1 000,101 1 010, 100 0 1 101, 000 1 010, 001 0 0 2 000, 110 1 100, 100 0 1110, 000 1 001, 100 unused 010, 010 100, 001

Each row of symbol groups and each column of symbol groups encodes athree-symbol 3-ary cyclic position code. (The Applicant's cyclicposition codes are described in U.S. Pat. No. 7,082,562, the contents ofwhich is herein incorporated by reference). The code consists of thecodeword (0, 1, 2) and its cyclic shifts. The code has a minimumdistance of 3, allowing a single symbol error to be corrected. The codesof an entire tag form a code with a minimum distance of 9, allowing 4symbol errors to be corrected. If additional symbols are visible withinthe field of view then they can be used for additional redundancy.

The translation code symbol in the middle of the codeword (i.e. 1) ismapped to a set of 2-6PPM symbol values that are each other's reverse,while the two translation code symbols at the ends of the codeword (i.e.0 and 2) are each mapped to a set of 2-6PPM symbol values that are thereverses of the 2-6PPM symbol values in the other set. Thus a 0 readupside-down (i.e. rotated 180 degrees) becomes a 2, and vice versa,while a 1 read upside-down remains a 1. This allows translation to bedetermined independently of rotation.

Each 2-6PPM symbol value and its reverse map to opposite direction codesymbol values. The vertical registration symbols of an entire tag encode9 symbols of a vertical direction code. This has a minimum distance of9, allowing 4 symbol errors to be corrected. The horizontal registrationsymbols of an entire tag encode 9 symbols of a horizontal directioncode. This has a minimum distance of 9, allowing 4 symbol errors to becorrected. If additional symbols are visible within the field of viewthen they can be used for additional redundancy. Any erasures detectedduring decoding of a translation code can also be used during decodingof a direction code, and vice versa. Together the orthogonal directioncodes allow the orientation of the tag to be determined.

The top left corner of an un-rotated tag is identified by a symbol groupwhose translation symbols are both zero and whose direction symbols areboth zero.

2.6.2 Flag Code

The flag symbol consists of one bit of data, and is encoded in eachvertical and horizontal registration symbol, as shown in Table 3.

The flag symbol is unique to a tag and is therefore coded redundantly ineach quadrant of the tag. Since the flag symbol is encoded in eachregistration symbol, it appears eight times within each quadrant. Eightsymbols form a code with a minimum distance of 8, allowing 3 errors tobe corrected. If additional symbols are visible within the field of viewthen they can be used for additional redundancy. Any erasures detectedduring decoding of translation and/or direction codes can also be usedduring decoding of the flag code, and vice versa.

2.6.3 Coordinate Data

The tag contains an x-coordinate codeword and a y-coordinate codewordused to encode the x and y coordinates of the tag respectively. Thecodewords are of a shortened 2⁴-ary (10, p) Reed-Solomon code, where pcan vary from 2 to 5. The tag therefore encodes between 8 and 20 bits ofinformation for each coordinate. This reduces to 7.8 to 19.5 bits oncebase-15 conversion occurs.

Each x coordinate codeword is replicated twice within the tag—in eachhorizontal half (“north” and “south”), and is constant within the columnof tags containing the tag. Likewise, each y coordinate codeword isreplicated twice within the tag—in each vertical half (“east” and“west”), and is constant within the row of tags containing the tag. Thisguarantees that an image of the tag pattern large enough to contain acomplete tag is guaranteed to contain a complete instance of eachcoordinate codeword, irrespective of the alignment of the image with thetag pattern. The instance of either coordinate codeword may consist offragments from different tags.

It should be noted that, in the present invention, some coordinatesymbols are not replicated and are placed on the dividing line betweenthe two halves of the tag. This arrangement saves tag space since thereare not two complete replications of each x-coordinate codeword and eachy-coordinate codeword contained in a tag. Since the field of view is atleast three macrodot units larger than the tag (as discussed in Section2.10), the coordinate symbols placed on the dividing line (having awidth 2 macrodot units) are still captured when the surface is imaged.Hence, each interaction with the coded surface still provides the taglocation.

The layout of the x-coordinate codeword is shown in FIG. 8. The layoutof the y-coordinate codeword is shown in FIG. 9. It can be seen thatx-coordinate symbols X4, X5, X6, X7, X8 and X9 are placed in a centralcolumn 310 of the tag 4, which divides the eastern half of the tag fromthe western half. Likewise, the y-coordinate symbols Y4, Y5, Y6, Y7, Y8and Y9 are placed in a central row 312 of the tag 4, which divides thenorthern half of the tag from the southern half.

The central column 310 and central row 312 each have a width q, whichcorresponds to a width of 2s, where s is the macrodot spacing.

2.6.4 Common Data

The tag contains four codewords A, B, C and D which encode informationcommon to a set of contiguous tags in a surface region. The codewordsare of a 2⁴-ary (15, 9) Reed-Solomon code. The tag therefore encodes upto 144 bits of information common to a set of contiguous tags. Thisreduces to 140 bits once base-15 conversion occurs.

The common codewords are replicated throughout a tagged region. Thisguarantees that an image of the tag pattern large enough to contain acomplete tag is guaranteed to contain a complete instance of each commoncodeword, irrespective of the alignment of the image with the tagpattern. The instance of each common codeword may consist of fragmentsfrom different tags.

The layout of the common codewords is shown in FIG. 10. The codewordshave the same layout, rotated 90 degree relative to each other.

2.6.5 Optional Data

The tag optionally contains a codeword E. This codeword may be used toencode a secret-key signature or a fragment of an embedded data object.These are discussed further in Sections 2.9.4 and Section 2.9.3respectively. The codeword is of a 2⁴-ary (15, 9) Reed-Solomon code.

The layout of the optional codeword is shown in FIG. 11.

2.6.6 Secret-Key Signature

The tag optionally contains an entire secret-key digital signaturecommon to a set of contiguous tags in a surface region. The signatureconsists of sixteen 2⁴-ary symbols (i.e. symbol E15 is also used). Thetag therefore optionally encodes up to 64 bits of secret-key signaturedata.

The signature is replicated throughout a tagged region. This guaranteesthat an image of the tag pattern large enough to contain a complete tagis guaranteed to contain a complete instance of the signature,irrespective of the alignment of the image with the tag pattern. Theinstance of the signature may consist of fragments from different tags.

The signature, if present, is encoded in the E codeword described inSection 2.6.5.

Digital signatures are discussed further in Section 2.9.4.

2.6.7 Complete Tag

FIG. 12 shows the layout of the data of a complete tag, with each symbolgroup comprising ten data symbols. The vertical and horizontalregistration symbols are not shown in FIG. 12.

2.7 Reed-Solomon Encoding 2.7.1 Reed-Solomon Codes

All data is encoded using a Reed-Solomon code defined over GF(2⁴). Thecode has a natural length n of 15. The dimension k of the code is chosento balance the error correcting capacity and data capacity of the code,which are (n−k)/2 and k symbols respectively.

The code may be punctured, by removing high-order redundancy symbols, toobtain a code with reduced length and reduced error correcting capacity.The code may also be shortened, by replacing high-order data symbolswith zeros, to obtain a code with reduced length and reduced datacapacity. Both puncturing and shortening can be used to obtain a codewith particular parameters. Shortening is preferred, where possible,since this avoids the need for erasure decoding.

The code has the following primitive polynominal:

p(x)=x ⁴ +x+1

The code has the following generator polynominal:

${g(x)} = {\prod\limits_{i = 1}^{n - k}\; \left( {x + \alpha^{t}} \right)}$

For a detailed description of Reed-Solomon codes, refer to Wicker, S. B.and V. K. Bhargava, eds., Reed-Solomon Codes and Their Applications,IEEE Press, 1994.

2.7.2 Codeword Organization

As shown in FIG. 13, redundancy coordinates r_(i) and data coordinatesd_(i) of the code are indexed from left to right according to the powerof their corresponding polynomial terms. The symbols X_(i) of a completecodeword are indexed from right to left to match the bit order of thedata. The bit order within each symbol is the same as the overall bitorder.

2.7.3 Code Instances

Table 4 defines the parameters of the different codes used in the tag.

TABLE 4 Codeword instances error- correcting data length dimensioncapacity capacity^(a) name description (n) (k) (symbols) (bits) X, Ycoordinate 10^(b) 5 2 19.5 codewords (see 4 3 15.6 Section 2.6.3) 3 311.7 2 4 7.8 A, B, common 15 9 3 35 C, D codewords (see Section 2.6.4) Eoptional codeword 15 9 3 35 (see Section 2.6.5) ^(a)takes into accountsymbol-wise conversion to base 15 to allow 2-6 PPM encoding^(b)shortened

2.8 Tag Coordinate Space

The tag coordinate space has two orthogonal axes labelled x and yrespectively. When the positive x axis points to the right then thepositive y axis points down.

The surface coding does not specify the location of the tag coordinatespace origin on a particular tagged surface, nor the orientation of thetag coordinate space with respect to the surface. This information isapplication-specific. For example, if the tagged surface is a sheet ofpaper, then the application which prints the tags onto the paper mayrecord the actual offset and orientation, and these can be used tonormalise any digital ink subsequently captured in conjunction with thesurface.

The position encoded in a tag is defined in units of tags. Byconvention, the tag position is taken to be the position of the top lefttarget in each tag.

2.9 Tag Information Content 2.9.1 Field Definitions

Table 5 defines the information fields embedded in the surface coding.

TABLE 5 Field Definitions field width (bits) description unique to tagactive area flag 1 A flag indicating whether the area^(a) immediatelysurrounding a tag intersects an active area. x coordinate 7.8-19.5 Theunsigned x coordinate of the tag^(b). y coordinate 7.8-19.5 The unsignedy coordinate of the tag^(b). common to tagged region encoding format 4The format of the encoding. 0: the present encoding. Other values arereserved region flags 12 Flags controlling the interpretation of regiondata (see Table 6). coordinate precision 2 A value (p) indicating theprecision of x and y coordinates according to the formula 8 + 4p.macrodot size ID 4 The ID of the macrodot size. coordinate width ID 2The ID of the width (w) of the x and y coordinates. 0: 7.8 bits 1:11.7bits 2: 15.6 bits 3: 19.5 bits region ID space ID 6 The ID of theregion ID space. 0: Netpage 1: EPC Other values are reserved for futureuse. region ID 96 The ID of the region containing the tags. secret-keysignature 64 An optional secret-key signature of the region. CRC (Cyclic16 A CRC^(c) of common tag data. Redundancy Check) ^(a)the diameter ofthe area, centered on the tag, is nominally 2.5 times the diagonal sizeof the tag; this is to accommodate the worst-case distance between thenib position and the imaged tag ^(b)allows a coordinate value range of857 mm (large enough for an A1 sheet) to 28.9 km for the nominal tagsize of 3.81 mm (based on the nominal macrodot size and 30 macrodots pertag) ^(c)CCITT CRC-16 [see ITU, Interface between Data TerminalEquipment (DTE) and Data Circuit-terminating Equipment (DCE) forterminals operating in the packet mode and connected to public datanetworks by dedicated circuit, ITU-T X.25 (10/96)], computed in bitorder on raw codeword data (see Table 4).

An active area is an area within which any captured input should beimmediately forwarded to the corresponding Netpage server 10 forinterpretation. This also allows the Netpage server 10 to signal to theuser that the input has had an immediate effect. Since the server hasaccess to precise region definitions, any active area indication in thesurface coding can be imprecise so long as it is inclusive.

TABLE 6 Region flags bit meaning 0 Region is interactive, i.e. x andy-coordinates are present. 1 Region is active, i.e. the entire region isan active area. Otherwise active areas are identified by individualtags' active area flags. 2 Region ID is not serialized^(a). 3 Region hassecret-key signature (see Section 2.9.4) 4 Region has embedded data. 5Embedded data is a public-key signature (see Sections 2.9.3 and 2.9.4).6 Page description is associated with region is public. Otherwise pagedescription is private. other Reserved for future use. Must be zero.^(a)For an EPC this means that the serial number is replaced by a layoutnumber, to allow the package design associated with a product to varyover time (see US 2007/0108285, the contents of which is hereinincorporated by reference).

2.9.2 Mapping of Fields to Codewords

Table 7 defines how the information fields map to codewords.

TABLE 7 Mapping of fields to codewords codeword codeword bits fieldwidth field bits X w − 1:0 x coordinate w all Y w − 1:0 y coordinate wall A 15:0 CRC^(a) 16 all 34:16 region ID 19 18:0  B 3:0 encoding format4 all 15:4  region flags 12 all 19:16 macrodot size ID 4 all 21:20coordinate width ID 2 all 27:22 region ID space ID 6 all 34:28 region ID7 25:19 C 34:0  region ID 35 60:26 D 34:0  region ID 35 95:61 E all datafragment 35 all E all^(b) secret-key signature 64 all ^(a)the CRC iscomputed in bit order on the data portions of the A, B, C and Dcodewords, in that order, excluding the CRC field itself ^(b)entirecodeword is used for data i.e. there is no redundancy

As shown in Table 7, codeword E either contains a data fragment or asecret-key signature. These are described in Section 2.9.3 and Section2.9.4 respectively. The secret-key signature is present in a particulartag if the “region has secret-key signature” flag in the region flags isset, and the tag's active area flag is set. The data fragment is presentif the “region contains embedded data” flag in the region flags is setand the tag's active area flag is not set.

When the region flags indicate that a particular codeword is absent thenthe codeword is not coded in the tag pattern, i.e. there are nomacrodots representing the codeword. This applies to the X, Y and Ecodewords.

2.9.3 Embedded Data Object

If the “region contains embedded data” flag in the region flags is setthen the surface coding contains embedded data. The embedded data isencoded in multiple contiguous tags' data fragments, and is replicatedin the surface coding as many times as it will fit.

The embedded data is encoded in such a way that a random and partialscan of the surface coding containing the embedded data can besufficient to retrieve the entire data. The scanning system reassemblesthe data from retrieved fragments, and reports to the user whensufficient fragments have been retrieved without error.

As shown in Table 8, each block has a data capacity of 170-bits. Theblock data is encoded in the data fragments of a contiguous group of sixtags arranged in a 3×2 rectangle.

The block parameters are as defined in Table 8. The E codeword of eachtag may encode a fragment of the embedded data.

TABLE 8 Block parameters parameter value description w 3 The width ofthe block, in tags h 2 The height of the block, in tags. b 170 The datacapacity of the block, in bits

If the E codeword of a particular tag does not contain a fragment of theembedded data, then the pen 101 can discover this implicitly by thefailure of the codeword to decode, or explicitly from the tag's activearea flag.

Data of arbitrary size may be encoded into a superblock consisting of acontiguous set of blocks, typically arranged in a rectangle. The size ofthe superblock may be encoded in each block.

The superblock is replicated in the surface coding as many times as itwill fit, including partially along the edges of the surface coding.

The data encoded in the superblock may include, for example, moreprecise type information, more precise size information, and moreextensive error detection and/or correction data.

2.9.4 Digital Signatures

As described in Section 2.6.6, a region may contain a digital signature.

If the <region has a secret-key signature> flag in the region flags isset, then the region has a secret-key digital signature. In an onlineenvironment the secret-key signature can be verified, in conjunctionwith the region ID, by querying a server with knowledge of thesecret-key signature or the corresponding secret key.

If the region contains embedded data and the <embedded data is apublic-key signature> flag in the region flag is set, then the surfacecoding contains an embedded public-key digital signature of the regionID.

In an online environment any number of signature fragments can be used,in conjunction with the region ID and optionally the secret-keysignature, to validate the public-key signature by querying a serverwith knowledge of the full public-key signature or the correspondingprivate key.

In an offline (or online) environment the entire public-key signaturecan be recovered by reading multiple tags, and can then be verifiedusing the corresponding public signature key. The actual length and typeof the signature are determined from the region ID during signaturevalidation i.e. typically from a previously-retrieved digital signatureassociated with a sequence of region IDs.

Digital signature verification is discussed in the Applicant's USPublication No. 2007/0108285, the contents of which are hereinincorporated by reference.

2.10 Tag Imaging and Decoding

The minimum imaging field of view required to guarantee acquisition ofdata from an entire tag has a diameter of 46.7s (i.e. ((3×10)+3)√2s),allowing for arbitrary rotation and translation of the surface coding inthe field of view. Notably, the imaging field of view does not have tobe large enough to guarantee capture of an entire tag—the arrangement ofthe data symbols within each tag ensures that a any square portion oflength (l+3s) captures the requisite information in full, irrespectiveof whether a whole tag is actually visible in the field-of-view. As usedherein, l is defined as the length of a tag.

In terms of imaging the coding pattern, the imaging field-of-view istypically a circle. Accordingly, the imaging field-of-view shouldpreferably have diameter of at least (l+3s)√2 and less than two tagdiameters. Importantly, the field-of-view is not required to be at leasttwo tag diameters, in contrast with prior art tag designs, because it isnot essential in the present invention to capture an entire tag in thefield of view.

The extra three macrodot units ensure that pulse-position modulatedvalues can be decoded from spatially coherent samples. Furthermore, theextra three macrodot units ensure that all requisite data symbols can beread with each interaction. These include the coordinate symbols from acentral column or row of a tag (see Section 2.6.3) having a width of 2s,and data symbols 304A extending from the perimeter edges of each tag byone macrodot unit (1s).

In the present context, a “tag diameter” is given to mean the length ofa tag diagonal.

Given a maximum macrodot spacing of 127 microns, this gives a requiredfield of view of 5.93 mm.

FIG. 14 shows a tag image processing and decoding process flow up to thestage of sampling and decoding the data codewords. Firstly, a raw image802 of the tag pattern is acquired (at 800), for example via an imagesensor such as a CCD image sensor, CMOS image sensor, or a scanninglaser and photodiode image sensor. The raw image 802 is then typicallyenhanced (at 804) to produce an enhanced image 806 with improvedcontrast and more uniform pixel intensities. Image enhancement mayinclude global or local range expansion, equalisation, and the like. Theenhanced image 806 is then typically filtered (at 808) to produce afiltered image 810. Image filtering may consist of low-pass filtering,with the low-pass filter kernel size tuned to obscure macrodots 302 butto preserve targets 301. The filtering step 808 may include additionalfiltering (such as edge detection) to enhance target features 301.Encoding of data codewords 304 using pulse position modulation (PPM)provides a more uniform coding pattern 3 than simple binary dot encoding(as described in, for example, U.S. Pat. No. 6,832,717).

Advantageously, this helps separate targets 301 from data areas, therebyallowing more effective low-pass filtering of the PPM-encoded datacompared to binary-coded data.

Following low-pass filtering, the filtered image 810 is then processed(at 812) to locate the targets 301. This may consist of a search fortarget features whose spatial inter-relationship is consistent with theknown geometry of the tag pattern. Candidate targets may be identifieddirectly from maxima in the filtered image 810, or may be the subject offurther characterization and matching, such as via their (binary orgrayscale) shape moments (typically computed from pixels in the enhancedimage 806 based on local maxima in the filtered image 810), as describedin U.S. Pat. No. 7,055,739, the contents of which is herein incorporatedby reference.

The identified targets 301 are then assigned (at 816) to a target grid818. Each cell of the grid 818 contains a symbol group 303, and severalsymbol groups will of course be visible in the image. At this stage,individual tags 4 will not be identifiable in the target grid 818, sincethe targets 301 do not demarcate one tag from another.

To allow macrodot values to be sampled accurately, the perspectivetransform of the captured image must be inferred. Four of the targets301 are taken to be the perspective-distorted corners of a square ofknown size in tag space, and the eight-degree-of-freedom perspectivetransform 822 is inferred (at 820), based on solving the well-understoodequations relating the four tag-space and image-space point pairs.Calculation of the 2D perspective transform is described in detail in,for example, Applicant's U.S. Pat. No. 6,832,717, the contents of whichis herein incorporated by reference.

Since each image will typically contain at least 16 targets arranged ina square grid, the accuracy of calculating the 2D perspective transformis improved compared to the Applicant's previous tag designs describedin, for example, U.S. Pat. No. 6,832,717. Hence, more accurate positioncalculation can be achieved with the tag design of the presentinvention.

The inferred tag-space to image-space perspective transform 822 is usedto project each known macrodot position in tag space into image space.Since all bits in the tags are represented by PPM-encoding, the presenceor absence of each macrodot 302 can be determined using a localintensity reference, rather than a separate intensity reference. Thus,PPM-encoding provides improved data sampling compared with pure binaryencoding.

The next stage determines the translation and orientation of the tag(s),or portions thereof, in the field of view relative to the target grid818. Two or more orthogonal registration symbols (‘VRS’ and ‘HRS’) aresampled (at 824), to allow decoding of the orthogonal translationcodewords and the orthogonal direction codewords.

Decoding of two or more orthogonal translation codewords (at 828) isused to determine the translation 830 of tags(s) in the field of viewrelative to the target grid 818. This enables alignment of the tags 4with the target grid 818, thereby allowing individual tag(s), orportions thereof, to be distinguished in the coding pattern 3 in thefield of view. Since each symbol group 303 contains orthogonalregistration symbols, multiple translation codes can be decoded toprovide robust translation determination. As described in Section 2.6.1, the translation code is a cyclic position code. Since each row andeach column of a tag contains M symbol groups, the code has minimumdistance M×M. This allows very robust determination of the alignment oftags 4 with the target grid 818. The alignment needs to be both robustand accurate since there are many possible alignments when each tag 4contains multiple symbol groups 303.

Likewise, at least two orthogonal direction codes are decoded (at 825)to provide the orientation 826. As described in Section 2.6.1, since Nvertical registration symbols in a tag form a vertical direction codewith minimum distance N, the vertical direction code is capable ofcorrecting (N−1)/2 errors. The horizontal direction code is similarlycapable of correcting (N−1)/2 errors using N horizontal registrationsymbols Hence, orientation determination is very robust and capable ofcorrecting errors, depending on the number of registration symbolssampled.

Once initial imaging and decoding has yielded the 2D perspectivetransform, the orientation, and the translation of tag(s) relative tothe target grid, the data codewords 304 can then be sampled and decoded(at 836) to yield the requisite decoded codewords 838.

Decoding of the data codewords 304 typically proceeds as follows:

-   -   sample common Reed-Solomon codewords    -   decode common Reed-Solomon codewords    -   verify common tag data CRC    -   on decode error flag bad region ID sample    -   determine encoding type, and reject unknown encoding    -   determine region flags    -   determine region ID    -   determine x and y coordinate widths from coordinate width ID    -   sample and decode x and y coordinate Reed-Solomon codewords    -   determine tag x-y location from codewords    -   determine nib x-y location from tag x-y location and perspective        transform taking into account macrodot size (from macrodot size        ID)    -   sample and decode four or more flag symbols to determine active        area flag    -   determine active area status of nib location with reference to        active area flag    -   encode region ID, nib x-y location, and nib active area status        in digital ink (“interaction data”)    -   route digital ink based on region flags

The skilled person will appreciate that the decoding sequence describedabove represents one embodiment of the present invention. It will, ofcourse, be appreciated that the interaction data sent from the pen 101to the netpage system may include other data e.g. digital signature (seeSection 2.9.4), pen mode (see US 2007/125860), orientation data, pen ID,nib ID etc.

An example of interpreting interaction data, received by the netpagesystem from the netpage pen 101, is discussed briefly above. A moredetailed discussion of how the netpage system may interpret interactiondata can be found in the Applicant's previously-filed applications (see,for example, US 2007/130117 and US 2007/108285, the contents of whichare herein incorporated by reference).

3. Netpage Pen 3.1 Functional Overview

The active sensing device of the netpage system may take the form of aclicker (for clicking on a specific position on a surface), a pointerhaving a stylus (for pointing or gesturing on a surface using pointerstrokes), or a pen having a marking nib (for marking a surface with inkwhen pointing, gesturing or writing on the surface). For a descriptionof various netpage sensing devices, reference is made to U.S. Pat. No.7,105,753; U.S. Pat. No. 7,015,901; U.S. Pat. No. 7,091,960; and USPublication No. 2006/0028459, the contents of each of which are hereinincorporated by reference.

It will be appreciated that the present invention may utilize anysuitable optical reader. However, the Netpage pen 400 will be describedherein as one such example.

The Netpage pen 400 is a motion-sensing writing instrument which worksin conjunction with a tagged Netpage surface (see Section 2). The penincorporates a conventional ballpoint pen cartridge for marking thesurface, an image sensor and processor for simultaneously capturing theabsolute path of the pen on the surface and identifying the surface, aforce sensor for simultaneously measuring the force exerted on the nib,and a real-time clock for simultaneously measuring the passage of time.

While in contact with a tagged surface, as indicated by the forcesensor, the pen continuously images the surface region adjacent to thenib, and decodes the nearest tag in its field of view to determine boththe identity of the surface, its own instantaneous position on thesurface and the pose of the pen. The pen thus generates a stream oftimestamped position samples relative to a particular surface, andtransmits this stream to the Netpage server 10. The sample streamdescribes a series of strokes, and is conventionally referred to asdigital ink (DInk). Each stroke is delimited by a pen down and a pen upevent, as detected by the force sensor. More generally, any dataresulting from an interaction with a Netpage, and transmitted to theNetpage server 10, is referred to herein as “interaction data”.

The pen samples its position at a sufficiently high rate (nominally100Hz) to allow a Netpage server to accurately reproduce hand-drawnstrokes, recognise handwritten text, and verify hand-written signatures.

The Netpage pen also supports hover mode in interactive applications. Inhover mode the pen is not in contact with the paper and may be somesmall distance above the surface of the paper (or other substrate). Thisallows the position of the pen, including its height and pose to bereported. In the case of an interactive application the hover modebehaviour can be used to move a cursor without marking the paper, or thedistance of the nib from the coded surface could be used for toolbehaviour control, for example an air brush function.

The pen includes a Bluetooth radio transceiver for transmitting digitalink via a relay device to a Netpage server. When operating offline froma Netpage server the pen buffers captured digital ink in non-volatilememory. When operating online to a Netpage server the pen transmitsdigital ink in real time.

The pen is supplied with a docking cradle or “pod”. The pod contains aBluetooth to USB relay. The pod is connected via a USB cable to acomputer which provides communications support for local applicationsand access to Netpage services.

The pen is powered by a rechargeable battery. The battery is notaccessible to or replaceable by the user. Power to charge the pen can betaken from the USB connection or from an external power adapter throughthe pod. The pen also has a power and USB-compatible data socket toallow it to be externally connected and powered while in use.

The pen cap serves the dual purpose of protecting the nib and theimaging optics when the cap is fitted and signalling the pen to leave apower-preserving state when uncapped.

3.2 Ergonomics and Layout

FIG. 15 shows a rounded triangular profile gives the pen 400 anergonomically comfortable shape to grip and use the pen in the correctfunctional orientation. It is also a practical shape for accommodatingthe internal components. A normal pen-like grip naturally conforms to atriangular shape between thumb 402, index finger 404 and middle finger406.

As shown in FIG. 16, a typical user writes with the pen 400 at a nominalpitch of about 30 degrees from the normal toward the hand 408 when held(positive angle) but seldom operates a pen at more than about 10 degreesof negative pitch (away from the hand). The range of pitch angles overwhich the pen 400 is able to image the pattern on the paper has beenoptimised for this asymmetric usage. The shape of the pen 400 helps toorient the pen correctly in the user's hand 408 and to discourage theuser from using the pen “upside-down”. The pen functions “upside-down”but the allowable tilt angle range is reduced.

The cap 410 is designed to fit over the top end of the pen 400, allowingit to be securely stowed while the pen is in use. Multi colour LEDsilluminate a status window 412 in the top edge (as in the apex of therounded triangular cross section) of the pen 400 near its top end. Thestatus window 412 remains un-obscured when the cap is stowed. Avibration motor is also included in the pen as a haptic feedback system(described in detail below).

As shown in FIG. 17, the grip portion of the pen has a hollow chassismolding 416 enclosed by a base molding 528 to house the othercomponents. The ink cartridge 414 for the ball point nib (not shown)fits naturally into the apex 420 of the triangular cross section,placing it consistently with the user's grip. This in turn providesspace for the main PCB 422 in the centre of the pen and for the battery424 in the base of the pen. By referring to FIG. 18A, it can be seenthat this also naturally places the tag-sensing optics 426 unobtrusivelybelow the nib 418 (with respect to nominal pitch). The nib molding 428of the pen 400 is swept back below the ink cartridge 414 to preventcontact between the nib molding 428 and the paper surface when the penis operated at maximum pitch.

As best shown in FIG. 18B, the imaging field of view 430 emerges througha centrally positioned IR filter/window 432 below the nib 418, and twonear-infrared illumination LEDs 434, 436 emerge from the two bottomcorners of the nib molding 428. Each LED 434, 436 has a correspondingillumination field 438, 440.

As the pen is hand-held, it may be held at an angle that causesreflections from one of the LED's that are detrimental to the imagesensor. By providing more than one LED, the LED causing the offendingreflections can be extinguished.

Specific details of the pen mechanical design can be found in USPublication No. 2006/0028459, the contents of which are hereinincorporated by reference.

3.3 Pen Feedback Indications

FIG. 19 is a longitudinal cross section through the centre-line if thepen 400 (with the cap 410 stowed on the end of the pen). The penincorporates red and green LEDs 444 to indicate several states, usingcolours and intensity modulation. A light pipe 448 on the LEDs 444transmit the signal to the status indicator window 412 in the tubemolding 416. These signal status information to the user includingpower-on, battery level, untransmitted digital ink, network connectionon-line, fault or error with an action, detection of an “active area”flag, detection of an “embedded data” flag, further data sampling torequired to acquire embedded data, acquisition of embedded datacompleted etc.

A vibration motor 446 is used to haptically convey information to theuser for important verification functions during transactions. Thissystem is used for important interactive indications that might bemissed due to inattention to the LED indicators 444 or high levels ofambient light. The haptic system indicates to the user when:

-   -   The pen wakes from standby mode    -   There is an error with an action    -   To acknowledge a transaction

3.4 Pen Optics

The pen incorporates a fixed-focus narrowband infrared imaging system.It utilizes a camera with a short exposure time, small aperture, andbright synchronised illumination to capture sharp images unaffected bydefocus blur or motion blur.

TABLE 6 Optical Specifications Magnification ^(~)0.225 Focal length of6.0 mm lens Viewing distance 30.5 mm Total track length 41.0 mm Aperturediameter 0.8 mm Depth of field _(.) ^(~)/6.5 mm Exposure time 200 usWavelength 810 nm Image sensor size 140 × 140 pixels Pixel size 10 umPitch range ^(~)15 to_(.) 45 deg Roll range ^(~)30 to_(.) 30 deg Yawrange 0 to 360 deg Minimum sampling 2.25 pixels per rate macrodotMaximum pen 0.5 m/s velocity ¹Allowing 70 micron blur radius²Illumination and filter ³Pitch, roll and yaw are relative to the axisof the pen

Cross sections showing the pen optics are provided in FIGS. 20A and 20B.An image of the Netpage tags printed on a surface 548 adjacent to thenib 418 is focused by a lens 488 onto the active region of an imagesensor 490. A small aperture 494 ensures the available depth of fieldaccommodates the required pitch and roll ranges of the pen 400.

First and second LEDs 434 and 436 brightly illuminate the surface 549within the field of view 430. The spectral emission peak of the LEDs ismatched to the spectral absorption peak of the infrared ink used toprint Netpage tags to maximise contrast in captured images of tags. Thebrightness of the LEDs is matched to the small aperture size and shortexposure time required to minimise defocus and motion blur.

A longpass IR filter 432 suppresses the response of the image sensor 490to any coloured graphics or text spatially coincident with imaged tagsand any ambient illumination below the cut-off wavelength of the filter432. The transmission of the filter 432 is matched to the spectralabsorption peak of the infrared ink to maximise contrast in capturedimages of tags. The filter also acts as a robust physical window,preventing contaminants from entering the optical assembly 470.

3.5 Pen Imaging System

A ray trace of the optic path is shown in FIG. 21. The image sensor 490is a CMOS image sensor with an active region of 140 pixels squared. Eachpixel is 10 μm squared, with a fill factor of 93%. Turning to FIG. 22,the lens 488 is shown in detail. The dimensions are:

-   -   D=3 mm    -   R1=3.593 mm    -   R2=15.0 mm    -   X=0.8246 mm    -   Y=1.0 mm    -   Z=0.25 mm

This gives a focal length of 6.15 mm and transfers the image from theobject plane (tagged surface 548) to the image plane (image sensor 490)with the correct sampling frequency to successfully decode all imagesover the specified pitch, roll and yaw ranges. The lens 488 is biconvex,with the most curved surface facing the image sensor. The minimumimaging field of view 430 required to guarantee acquisition ofsufficient tag data with each interaction is dependent on the specificcoding pattern. The required field of view for the coding pattern of thepresent invention is described in Section 2.10.

The required paraxial magnification of the optical system is defined bythe minimum spatial sampling frequency of 2.25 pixels per macrodot forthe fully specified tilt range of the pen 400, for the image sensor 490of 10 μm pixels. Typically, the imaging system employs a paraxialmagnification of 0.225, the ratio of the diameter of the inverted imageat the image sensor to the diameter of the field of view at the objectplane, on an image sensor 490 of minimum 128×128 pixels. The imagesensor 490 however is 140×140 pixels, in order to accommodatemanufacturing tolerances. This allows up to ±120 μm (12 pixels in eachdirection in the plane of the image sensor) of misalignment between theoptical axis and the image sensor axis without losing any of theinformation in the field of view.

The lens 488 is made from Poly-methyl-methacrylate (PMMA), typicallyused for injection moulded optical components. PMMA is scratchresistant, and has a refractive index of 1.49, with 90% transmission at810 nm. The lens is biconvex to assist moulding precision and features amounting surface to precisely mate the lens with the optical barrelmolding 492.

A 0.8 mm diameter aperture 494 is used to provide the depth of fieldrequirements of the design.

The specified tilt range of the pen is 15.0 to 45.0 degree pitch, with aroll range of 30.0 to 30.0 degrees. Tilting the pen through itsspecified range moves the tilted object plane up to 6.3 mm away from thefocal plane. The specified aperture thus provides a corresponding depthof field of 6.5 mm, with an acceptable blur radius at the image sensorof 16 μm.

Due to the geometry of the pen design, the pen operates correctly over apitch range of 33.0 to 45.0 degrees.

Referring to FIG. 23, the optical axis 550 is pitched 0.8 degrees awayfrom the nib axis 552. The optical axis and the nib axis converge towardthe paper surface 548. With the nib axis 552 perpendicular to the paper,the distance A between the edge of the field of view 430 closest to thenib axis and the nib axis itself is 1.2 mm.

The longpass IR filter 432 is made of CR-39, a lightweight thermosetplastic heavily resistant to abrasion and chemicals such as acetone.Because of these properties, the filter also serves as a window. Thefilter is 1.5 mm thick, with a refractive index of 1.50. Each filter maybe easily cut from a large sheet using a CO₂ laser cutter.

3.6 Electronics Design

TABLE 3 Electrical Specifications Processor ARM7 (Atmel AT91FR40162)running at 80 MHz with 256 kB SRAM and 2 MB flash memory Digital inkstorage 5 hours of writing capacity Bluetooth 1.2 Compliance USBCompliance 1.1 Battery standby 12 hours (cap off), >4 weeks (cap on)time Battery writing 4 hours of cursive writing (81% pen down, timeassuming easy offload of digital ink) Battery charging 2 hours timeBattery Life Typically 300 charging cycles or 2 years (whichever occursfirst) to 80% of initial capacity. Battery ~340 mAh at 3.7 V,Lithium-ion Polymer Capacity/Type (LiPo)

FIG. 24 is a block diagram of the pen electronics. The electronicsdesign for the pen is based around five main sections. These are:

-   -   the main ARM7 microprocessor 574,    -   the image sensor and image processor 576,    -   the Bluetooth communications module 578,    -   the power management unit IC (PMU) 580 and    -   the force sensor microprocessor 582.

3.6.1 Microprocessor

The pen uses an Atmel AT91FR40162 microprocessor (see Atmel, AT91 ARMThumb Microcontrollers—AT91FR40162 Preliminary,http://www.keil.com/dd/docs/datashts/atmel/at91fr40162.pdf) running at80 MHz. The AT91FR40162 incorporates an ARM7 microprocessor, 256 kBytesof on-chip single wait state SRAM and 2 MBytes of external flash memoryin a stack chip package.

-   -   This microprocessor 574 forms the core of the pen 400. Its        duties include:        -   setting up the Jupiter image sensor 584,        -   decoding images of Netpage coding pattern (see Section            2.10), with assistance from the image processing features of            the image sensor 584, for inclusion in the digital ink            stream along with force sensor data received from the force            sensor microprocessor 582,        -   setting up the power management IC (PMU) 580,        -   compressing and sending digital ink via the Bluetooth            communications module 578, and        -   programming the force sensor microprocessor 582.

The ARM7 microprocessor 574 runs from an 80 MHz oscillator. Itcommunicates with the Jupiter image sensor 576 using a UniversalSynchronous Receiver Transmitter (USRT) 586 with a 40 MHz clock. TheARM7 574 communicates with the Bluetooth module 578 using a UniversalAsynchronous Receiver Transmitter (UART) 588 running at 115.2 kbaud.

Communications to the PMU 580 and the Force Sensor microprocessor (FSP)582 are performed using a Low Speed Serial bus (LSS) 590. The LSS isimplemented in software and uses two of the microprocessor's generalpurpose IOs.

The ARM7 microprocessor 574 is programmed via its JTAG port.

3.6.2 Image Sensor

The ‘Jupiter’ Image Sensor 584 (see US Publication No. 2005/0024510, thecontents of which are incorporated herein by reference) contains amonochrome sensor array, an analogue to digital converter (ADC), a framestore buffer, a simple image processor and a phase lock loop (PLL). Inthe pen, Jupiter uses the USRT's clock line and its internal PLL togenerate all its clocking requirements. Images captured by the sensorarray are stored in the frame store buffer. These images are decoded bythe ARM7 microprocessor 574 with help from the ‘Callisto’ imageprocessor contained in Jupiter. The Callisto image processor performs,inter alia, low-pass filtering of captured images (see Section 2.10 andUS Publication No. 2005/0024510) before macrodot sampling and decodingby the microprocessor 574.

Jupiter controls the strobing of two infrared LEDs 434 and 436 at thesame time as its image array is exposed. One or other of these twoinfrared LEDs may be turned off while the image array is exposed toprevent specular reflection off the paper that can occur at certainangles.

3.6.3 Bluetooth Communications Module

The pen uses a CSR BlueCore4-External device (see CSR,BlueCore4—External Data Sheet rev c, 6 Sep. 2004) as the Bluetoothcontroller 578. It requires an external 8 Mbit flash memory device 594to hold its program code. The BlueCore4 meets the Bluetooth v1.2specification and is compliant to v0.9 of the Enhanced Data Rate (EDR)specification which allows communication at up to 3 Mbps.

A 2.45 GHz chip antenna 486 is used on the pen for the Bluetoothcommunications.

The BlueCore4 is capable of forming a UART to USB bridge. This is usedto allow USB communications via data/power socket 458 at the top of thepen 456.

Alternatives to Bluetooth include wireless LAN and PAN standards such asIEEE 802.11 (Wi-Fi) (see IEEE, 802.11 Wireless Local Area Networks,http://grouper.ieee.org/groups/802/11/index.html), IEEE 802.15 (seeIEEE, 802.15 Working Group for WPAN,http://grouper.ieee.org/groups/802/15/index.html), ZigBee (see ZigBeeAlliance, http://www.zigbee.org), and WirelessUSB Cypress (seeWirelessUSB LR 2.4-GHz DSSS Radio SoC,http://www.cypress.com/cfuploads/img/products/cywusb6935.pdf), as wellas mobile standards such as GSM (see GSM Association,http://www.gsmworld.com/index.shtml), GPRS/EDGE, GPRS Platform,http://www.gsmworld.com/technology/gprs/index.shtml), CDMA (see CDMADevelopment Group, http://www.edg.org, and Qualcomm,http://www.qualcomm.com), and UMTS (see 3rd Generation PartnershipProject (3GPP), littl:/www.3gpp.org).

3.6.4 Power Management Chip

The pen uses an Austria Microsystems AS3603 PMU 580 (see AustriaMicrosystems, AS3603 Multi-Standard Power Management Unit Data Sheet v2.0). The PMU is used for battery management, voltage generation, powerup reset generation and driving indicator LEDs and the vibrator motor.

The PMU 580 communicates with the ARM7 microprocessor 574 via the LSSbus 590.

3.6.5 Force Sensor Subsystem

The force sensor subsystem comprises a custom Hokuriku force sensor 500(based on Hokuriku, HFD-500 Force Sensor,http://www.hdk.cojp/pdf/eng/e1381AA.pdf), an amplifier and low passfilter 600 implemented using op-amps and a force sensor microprocessor582.

The pen uses a Silicon Laboratories C8051F330 as the force sensormicroprocessor 582 (see Silicon Laboratories, C8051F330/1 MCUData Sheet,rev 1.1). The C8051F330 is an 8051 microprocessor with on chip flashmemory, 10 bit ADC and 10 bit DAC. It contains an internal 24.5 MHzoscillator and also uses an external 32.768 kHz tuning fork.

The Hokuriku force sensor 500 is a silicon piezoresistive bridge sensor.An op-amp stage 600 amplifies and low pass (anti-alias) filters theforce sensor output. This signal is then sampled by the force sensormicroprocessor 582 at 5 kHz.

Alternatives to piezoresistive force sensing include capacitive andinductive force sensing (see Wacom, “Variable capacity condenser andpointer”, US Patent Application 20010038384, filed 8 Nov. 2001, andWacom, Technology, http://www.wacom-components.com/english/tech.asp).

The force sensor microprocessor 582 performs further (digital) filteringof the force signal and produces the force sensor values for the digitalink stream. A frame sync signal from the Jupiter image sensor 576 isused to trigger the generation of each force sample for the digital inkstream. The temperature is measured via the force sensormicroprocessor's 582 on chip temperature sensor and this is used tocompensate for the temperature dependence of the force sensor andamplifier. The offset of the force signal is dynamically controlled byinput of the microprocessor's DAC output into the amplifier stage 600.

The force sensor microprocessor 582 communicates with the ARM7microprocessor 574 via the LSS bus 590. There are two separate interruptlines from the force sensor microprocessor 582 to the ARM7microprocessor 574. One is used to indicate that a force sensor sampleis ready for reading and the other to indicate that a pen down/up eventhas occurred.

The force sensor microprocessor flash memory is programmed in-circuit bythe ARM7 microprocessor 574.

The force sensor microprocessor 582 also provides the real time clockfunctionality for the pen 400. The RTC function is performed in one ofthe microprocessor's counter timers and runs from the external 32.768kHz tuning fork. As a result, the force sensor microprocessor needs toremain on when the cap 472 is on and the ARM7 574 is powered down. Hencethe force sensor microprocessor 582 uses a low power LDO separate fromthe PMU 580 as its power source. The real time clock functionalityincludes an interrupt which can be programmed to power up the ARM7 574.

The cap switch 602 is monitored by the force sensor microprocessor 582.When the cap assembly 472 is taken off (or there is a real time clockinterrupt), the force sensor microprocessor 582 starts up the ARM7 572by initiating a power on and reset cycle in the PMU 580.

3.7 Pen Software

The Netpage pen software comprises that software running onmicroprocessors in the Netpage pen 400 and Netpage pod.

The pen contains a number of microprocessors, as detailed in Section3.6. The Netpage pen software includes software running on the AtmelARM7 CPU 574 (hereafter CPU), the Force Sensor microprocessor 582, andalso software running in the VM on the CSR BlueCore Bluetooth module 578(hereafter pen BlueCore). Each of these processors has an associatedflash memory which stores the processor specific software, together withsettings and other persistent data. The pen BlueCore 578 also runsfirmware supplied by the module manufacturer, and this firmware is notconsidered a part of the Netpage pen software.

The pod contains a CSR BlueCore Bluetooth module (hereafter podBlueCore). The Netpage pen software also includes software running inthe VM on the pod BlueCore.

As the Netpage pen 400 traverses a Netpage tagged surface 548, a streamof correlated position and force samples are produced. This stream isreferred to as DInk. Note that DInk may include samples with zero force(so called “Hover DInk”) produced when the Netpage pen is in proximityto, but not marking, a Netpage tagged surface.

The CPU component of the Netpage pen software is responsible for DInkcapture, tag image processing and decoding (in conjunction with theJupiter image sensor 576), storage and offload management, hostcommunications, user feedback and software upgrade. It includes anoperating system (RTOS) and relevant hardware drivers. In addition, itprovides a manufacturing and maintenance mode for calibration,configuration or detailed (non-field) fault diagnosis. The Force Sensormicroprocessor 582 component of the Netpage pen software is responsiblefor filtering and preparing force samples for the main CPU. The penBlueCore VM software is responsible for bridging the CPU UART 588interface to USB when the pen is operating in tethered mode. The penBlueCore VM software is not used when the pen is operating in Bluetoothmode.

The pod BlueCore VM software is responsible for sensing when the pod ischarging a pen 400, controlling the pod LEDs appropriately, andcommunicating with the host PC via USB.

For a detailed description of the software modules, reference is made toUS Publication No. 2006/0028459, the contents of which are hereinincorporated by reference.

The present invention has been described with reference to a preferredembodiment and number of specific alternative embodiments. However, itwill be appreciated by those skilled in the relevant fields that anumber of other embodiments, differing from those specificallydescribed, will also fall within the spirit and scope of the presentinvention. Accordingly, it will be understood that the invention is notintended to be limited to the specific embodiments described in thepresent specification, including documents incorporated bycross-reference as appropriate. The scope of the invention is onlylimited by the attached claims.

1. A method of imaging a coding pattern disposed on a surface of asubstrate, said method comprising the steps of: (a) capturing an imageof a portion of said coding pattern, said coding pattern comprising: aplurality of target elements defining a target grid, said target gridcomprising a plurality of cells, wherein neighboring cells share targetelements; a plurality of data elements contained in each cell; and aplurality of tags, each tag being defined by a plurality of contiguouscells, each tag comprising respective tag data encoded by data elements,wherein each cell comprises at least one registration symbol encoded bya respective set of said data elements, said set of data elementsidentifying a translation of said cell relative to a tag containing saidcell; (b) sampling one or more registration symbols contained in saidimaged portion; (c) mapping each set of data elements to a firsttranslation code symbol value when read in a first orientation or asecond translation symbol code value when read in a second orientation;(d) determining the translation from the mapped set of data elements;and (e) using the translation to sample and decode said tag data.
 2. Themethod of claim 1, wherein second orientation is rotated 180 degreesfrom said first orientation.
 3. The method of claim 1, wherein said setof data elements is represented by multi-pulse position modulation. 4.The method of claim 3, wherein said set of data elements consists of mmacrodots, each of said macrodots occupying a respective position from aplurality of predetermined possible positions p within said cell, therespective positions of said macrodots representing one of a pluralityof possible registration symbol values for said registration symbol. 5.The substrate of claim 4, wherein m is 2 and p is 6 so as to provide 15possible registration symbol values.
 6. The method of claim 4, wherein aplurality of said registration symbol values are mapped to eachtranslation code value.
 7. The method of claim 4, wherein said secondorientation reverses each registration symbol value with respect to saidfirst orientation.
 8. The method of claim 7, wherein each tag containsan odd number of cells aligned in a row or column, and wherein aregistration symbol contained in a central cell of said row or column isrepresented by a registration symbol value which maps to identical firstand second translation code symbol values.
 9. The method of claim 7,wherein each tag contains a plurality of cells aligned in a row orcolumn, each row or column having a first cell at one end and a secondcell at an opposite end, said first cell containing a first registrationsymbol represented by a first registration symbol value which maps to afirst translation symbol code value in said first orientation, saidsecond cell containing a second registration symbol represented by asecond registration symbol value which maps to a second translation codesymbol value in said first orientation, wherein said first translationcode symbol value and said second translation code symbol value areinterchanged in said second orientation.
 10. The method of claim 1,further comprising the step of decoding, from each registration symbol,at least one of: an orientation of a layout of said tag data withrespect to said target grid; a direction component of said orientation;and a flag code.
 11. A system for imaging a coding pattern disposed on asurface of a substrate, said system comprising: (A) said substrate,wherein said coding pattern comprises: a plurality of target elementsdefining a target grid, said target grid comprising a plurality ofcells, wherein neighboring cells share target elements; a plurality ofdata elements contained in each cell; and a plurality of tags, each tagbeing defined by a plurality of contiguous cells, each tag comprisingrespective tag data encoded by data elements, wherein each cellcomprises at least one registration symbol encoded by a respective setof said data elements, said set of data elements identifying atranslation of said cell relative to a tag containing said cell; and (B)an optical reader comprising: an image sensor for capturing an image ofa portion of said coding pattern; and a processor configured for: (i)sampling one or more registration symbols contained in said imagedportion; (ii) mapping each set of data elements to a first translationcode symbol value when read in a first orientation or a secondtranslation code symbol value when read in a second orientation; (iii)determining the translation from the mapped set of data elements; and(iv) using the translation to sample and decode said tag data.
 12. Thesystem of claim 11, wherein second orientation is rotated 180 degreesfrom said first orientation.
 13. The system of claim 11, wherein saidset of data elements is represented by multi-pulse position modulation.14. The system of claim 13, wherein said set of data elements consistsof m macrodots, each of said macrodots occupying a respective positionfrom a plurality of predetermined possible positions p within said cell,the respective positions of said macrodots representing one of aplurality of possible registration symbol values for said registrationsymbol.
 15. The system of claim 14, wherein m is 2 or more and p>m. 16.The system of claim 14, wherein a plurality of said registration symbolvalues are mapped to each translation code value.
 17. The system ofclaim 14, wherein said second orientation reverses each registrationsymbol value with respect to said first orientation.
 18. The system ofclaim 17, wherein each tag contains an odd number of cells aligned in arow or column, and wherein a registration symbol contained in a centralcell of said row or column is represented by a registration symbol valuewhich maps to identical first and second translation code symbol values.19. The system of claim 17, wherein each tag contains a plurality ofcells aligned in a row or column, each row or column having a first cellat one end and a second cell at an opposite end, said first cellcontaining a first registration symbol represented by a firstregistration symbol value which maps to a first translation symbol codevalue in said first orientation, said second cell containing a secondregistration symbol represented by a second registration symbol valuewhich maps to a second translation code symbol value in said firstorientation, wherein said first translation code symbol value and saidsecond translation code symbol value are interchanged in said secondorientation.
 20. The system of claim 11, wherein said processor isfurther configured for decoding, from each registration symbol, at leastone of: an orientation of a layout of said tag data with respect to saidtarget grid; a direction component of said orientation; and a flag code.